As a conventional electronic watch, there are electronic watches provided with a built-in electric power generator for converting external energy such as photovoltaic energy, thermal energy, mechanical energy, and the like into electric energy (generation of power), and driving watch means by this electric energy, thereby executing time display operation.
In an electronic watch having a primary cell such as a silver cell or a lithium cell as a power supply, there is generally provided means for monitoring the amount of power stored in the power supply cell, for example, when a cell voltage is detected and the detected cell voltage falls to a value lower than a given value, hands such as a second hand of the time display means are caused to do a special hand operation such as two-second hand operation which is different from the normal hand operation, thereby facilitating the replacement of the cell.
Further, in an electronic watch having a power supply which is obtained by converting external energy into power, and charging the power in a capacitor having large capacity such as an electronic double-layer capacitor, the charging voltage is detected, and then a second hand is fast forwarded in response to the detected voltage value, thereby generally displaying an operable time of the watch by the amount of forward motion of the second hand.
In such conventional electronic watches, the cell voltage or the charging voltage is detected to display a lowering of the voltage. Accordingly, it is impossible to exactly know the amount of stored power.
For example, in the case of an electronic watch having a silver cell as a power supply, discharging characteristics of the silver cell are as shown in a graph in FIG. 17, wherein a curve E showing the cell voltage always represents a constant voltage value of 1.5 volts during 1 to 2 years. However, the amount of stored electric charge is largely reduced as shown by a curve Q. Accordingly, it is not possible to determine the stored power of the silver cell from the cell voltage until the life of the cell almost expires.
The amount of stored power cannot be conjectured by detecting the cell voltage until the life of the cell almost expires to lower the cell voltage to about 1.3 volts. However, as shown in FIG. 17, the quantity Q32 of electric charge at that time is very small, and the amount of stored power which is the product of the quantity Q of electric charge and the power supply voltage E is very small to the extent that the electronic watch operates only for a further couple of days.
Accordingly, the stored power becomes less sooner than the operating voltage of the electronic watch indicates, namely V32=0.9 volt, and hence users have little time to replace the cell, so that the electronic watch inconveniently stops.
In an electronic watch for converting external energy into power for charging the power in a capacitor having large capacity, the amount of charged power is considered to be proportional to the capacity value of the large capacitor times the charging voltage squared, and hence it is possible to conjecture the charging capacity from the charging voltage. However, the operable time of the electronic watch can be known as a general rule.
Although it is not impossible to determine the amount of stored power by use of an operation circuit including a multiplier, it requires a complex circuit arrangement and the power consumption increases, which is not practical for a wrist watch. Even in this case, since the operable time of the electronic watch can be detected for only several tens of hours before power exhaustion at the most, it is necessary for users to quickly charge the cell when the electronic watch stops.
The present invention has been made to improve upon the problems set forth above, and it is an object of the present invention to provide an electronic watch capable of always exactly monitoring the amount of stored power of the charging means or power supply cell thereof.